Forming a membrane having curved features

ABSTRACT

Processes for making a membrane having a curved feature are disclosed. Recesses each in the shape of a reversed, truncated pyramid are formed in a planar substrate surface by KOH etching through a mask. An oxide layer is formed over the substrate surface. The oxide layer can be stripped leaving rounded corners between different facets of the recesses in the substrate surface, and the substrate surface can be used as a profile-transferring substrate surface for making a membrane having concave curved features. Alternatively, a handle layer is attached to the oxide layer and the substrate is removed until the backside of the oxide layer becomes exposed. The exposed backside of the oxide layer includes curved portions protruding away from the handle layer, and can provide a profile-transferring substrate surface for making a membrane having convex curved features.

TECHNICAL FIELD

This specification relates to fabrication of MEMS devices.

BACKGROUND

Many MEMS devices include piezoelectric actuators that deflect underapplied electric voltages. Examples of such devices include fluidejection systems that eject fluid in response to the actuation of apiezoelectric actuator connected to a fluid path. A printhead module inan ink jet printer is an example of a fluid ejection system. A printheadmodule typically has a line or an array of nozzles with a correspondingarray of ink paths and associated actuators, and drop ejection from eachnozzle can be independently controlled by one or more controllers.

A printhead module can include a semiconductor printhead die that isetched to define a fluid path that includes a pumping chamber. Apiezoelectric actuator can be formed on one side of the pumping chamberand, in operation, can flex in response to a driving voltage signal todrive fluid along the ink path. The piezoelectric actuator includes alayer of piezoelectric material that changes geometry (i.e., actuates)in response to the driving voltage applied across the piezoelectriclayer by a pair of opposing electrodes.

A piezoelectric element that is curved, such as a dome-shaped ordent-shaped piezoelectric membrane, can produce a larger displacementunder a given driving voltage as compared to a flat piezoelectricelement of similar lateral dimensions. Since the magnitude of thepiezoelectric displacement affects the driving voltage that is requiredto eject fluid droplets of a desired drop volume, and hence, affects thepower efficiency of the printhead module, piezoelectric actuators havingcurved piezoelectric membranes have been proposed. Various fabricationmethods have been proposed to produce piezoelectric membranes that arecurved or have curved features.

SUMMARY

This specification describes technologies related to MEMS fabricationprocesses for producing membranes having curved features.

When a thin layer of material is uniformly deposited on aprofile-transferring substrate surface, the layer of material assumesshapes that conform to the profile of the profile-transferring substratesurface.

To prepare the profile-transferring substrate surface for making amembrane having a concave feature, first, a flat-bottomed recess (i.e.,a recess in the shape of an inverted, truncated pyramid) is formed in aplanar top surface of a semiconductor substrate by anisotropic etchingthrough a patterned mask. Then, the top surface of the semiconductorsubstrate is oxidized to form an oxide layer that covers both the insideof the recess and the planar substrate surface surrounding the recess.The oxidation process rounds the abrupt corners within the recess. Theoxide layer can then be stripped leaving rounded intersections betweenthe bottom and the slanted sidewalls of the recess, and between adjacentsidewalls of the recess. After the oxide layer is stripped, thesubstrate surface can be used as the profile-transferring substratesurface for making membranes having a concave feature (e.g., a dent)formed therein.

Alternatively, to prepare the profile-transferring substrate surface formaking a membrane having a convex feature, after the top surface of thesemiconductor substrate is oxidized to form the oxide layer that coversboth the inside of the recess and the planar substrate surfacesurrounding the recess, a handle layer can be attached to the topsurface of the oxide layer (in contact with only the planar portion ofthe oxide layer). The curved portions of the oxide layer protrude awayfrom the handle layer and form domes relative to the handle layer. Then,the semiconductor substrate can be removed (e.g., by etching) until theback side of the oxide layer (i.e., the side of the oxide layer adjacentto the substrate) are exposed. The exposed back side of the oxide layercan be used as the profile-transferring substrate surface for makingmembranes having convex features (i.e., domes) formed therein.

In one aspect, a process for forming a membrane having a curved featureincludes the actions of: oxidizing a top surface of a semiconductorsubstrate to form an oxide layer, the top surface of the semiconductorsubstrate having a recess formed therein, the recess having a flat endwall and multiple flat, slanted sidewalls adjacent to the side wall, theoxide layer including a first portion on the end wall of the recess, asecond portion on the slanted sidewalls of the recess, and a thirdportion on the top surface of the semiconductor substrate around anopening of the recess in the top surface of the semiconductor substrate;and stripping the oxide layer to leave respective rounded intersectionsbetween the end wall and each slanted sidewall of the recess and betweenadjacent sidewalls of the recess.

In some implementations, the process further include: after thestripping, depositing a layer of a first material on the top surface ofthe semiconductor substrate, the layer of the first material covering atleast the end wall and the multiple slanted sidewalls of the recess andthe respective rounded intersections; and removing at least a portion ofthe semiconductor substrate from a bottom surface of the semiconductorsubstrate to expose a bottom surface of the layer of the first materialin at least areas previously covering the end wall of the recess and alower portion of each sidewall adjacent to the end wall.

In some implementations, the process further include: etching the recessin the top surface of the semiconductor substrate through a mask usingan anisotropic etchant.

In some implementations, the process further include: prior to theetching, forming the mask on the top surface of semiconductor substrate,the mask having an opening through which the top surface of thesemiconductor substrate is exposed, and the opening in the mask havingmultiple straight edges.

In some implementations, the opening in the mask is of a rectangularshape.

In some implementations, the anisotropic etchant is KOH.

In another aspect, a process for forming a membrane having a curvedfeature include the actions of: oxidizing a top surface of asemiconductor substrate to form an oxide layer, the top surface of thesemiconductor substrate having a recess formed therein, the recesshaving a flat end wall and multiple flat, slanted sidewalls adjacent tothe end wall, the oxide layer including a first portion on the end wallof the recess, a second portion on the slanted sidewalls of the recess,and a third portion on the top surface of the semiconductor substratearound an opening of the recess in the top surface of the semiconductorsubstrate; bonding a handle layer to a first side of the oxide layer,the handle layer in contact with the third portion of the oxide layer;and removing at least part of the semiconductor substrate to expose asecond side of the oxide layer opposite to the first side, in areas ofthe first portion and at least part of the second portion adjacent tothe first portion of the oxide layer.

In some implementations, the process further include: depositing a layerof a first material on the second side the oxide layer over at least thefirst portion and the at least part of the second portion of the oxidelayer.

In some implementations, the process further include: after depositingthe layer of the first material, removing at least part of the handlelayer to re-expose the first side of the oxide layer in at least thefirst portion and the second portion of the oxide layer.

In some implementations, the process further include: after removing atleast the part of the handle layer, removing at least the first portionand the at least part of the second portion of the oxide layer to exposea portion of the layer of the first material previously deposited on theremoved portions of the oxide layer.

In some implementations, the process further include: etching the recessin the top surface of the semiconductor substrate through a mask usingan anisotropic etchant.

In some implementations, the process further include: prior to theetching, forming a mask on the top surface of semiconductor substrate,the mask having an opening through which the top surface of thesemiconductor substrate is exposed, and the opening in the mask havingmultiple straight edges.

In some implementations, the opening in the mask is of a rectangularshape.

In some implementations, the anisotropic etchant is KOH.

In another aspect, an apparatus includes a base wafer; and a membraneattached to a top surface of the base wafer, where the membrane is acontinuous layer and includes a planar portion and a curved portionprotruding from the planar portion, the curved portion includes a flat,end wall parallel to the top surface of the base wafer and multipleflat, slanted sidewalls adjacent to the end wall, and respectiveintersections between the end wall and each of the slanted sidewalls andrespective intersections between each pair of adjacent sidewalls arerounded.

In some implementations, the curved portion protrudes away from the basewafer.

In some implementations, the base wafer includes an aperture and thecurved portion protrudes into the aperture in the base wafer.

In some implementations, the membrane is a silicon oxide membrane.

In some implementations, a height of the curved portion is between 5-10microns.

In some implementations, a lateral dimension of the curve portion isbetween 100-200 microns.

In some implementations, a thickness of the membrane is between 1-3microns.

In some implementations, the apparatus further includes: a piezoelectricactuation assembly formed over a top surface of the membrane, thepiezoelectric actuation assembly including a curved portion thatconforms to a profile of the curved portion of the membrane.

In some implementations, the piezoelectric actuation assembly includinga conductive reference electrode layer, a conductive drive electrodelayer, and a sputtered piezoelectric layer between the referenceelectrode layer and the drive electrode layer.

In some implementations, the membrane is of substantially uniformthickness.

In some implementations, at least part of the curved portion is thinnerthan the planar portion of the membrane.

Particular implementations of the subject matter described in thisspecification can be implemented to realize one or more of the followingadvantages.

A profile-transferring substrate surface having curved features can beformed via a series of standard MEMS fabrication processes. The sizes,shapes, and locations of the curved features formed in theprofile-transferring substrate surface are uniform and controllable. Byusing the profile-transferring substrate surface produced according tothe methods disclosed in this specification, membranes of variousmaterials can be formed over the profile-transferring substrate surface,where each membrane also assumes curved features conforming to thoseexisting in the profile-transferring substrate surface.

The grain structures of a membrane formed by material deposition overthe profile-transferring substrate surface, such as a piezoelectricmembrane deposited by sputtering, can be more uniform in size anddistribution and be more uniformly aligned than those achievable byinjection molding or mechanical means. The more uniform and alignedgrain structures can help improve the lifetime of the membrane duringrepeated actuations.

Curved piezoelectric actuators and transducers can be formed ofmembranes (e.g., piezoelectric membrane and conductive membranes) havingcurved features. When the curved features have rounded and smoothtransitions between different flat portions of the curved features, thecorresponding piezoelectric actuators and transducers can be moredurable and can better withstand the stress induced during theoperations of the actuators and transducers.

The processes described in this specification can be used to form adurable, highly efficient, compact, and high resolution integratedpiezoelectric actuator assembly or piezoelectric transducer array thatinclude curved piezoelectric elements.

The details of one or more implementations of the subject matterdescribed in this specification are set forth in the accompanyingdrawings and the description below. Other features, aspects, andadvantages of the subject matter will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1M illustrate two example processes for forming a membranehaving curved features. FIGS. 1A-1D followed by FIGS. 1E-1G illustratethe example process for making a membrane having concave features (e.g.,dents). FIGS. 1A-1D followed by FIGS. 1H-1M illustrate the exampleprocess for making a membrane having convex features (e.g., domes).

FIG. 2A is a schematic cross-sectional view of a printhead die in anexample fluid ejection module having a concave piezoelectric actuator.

FIG. 2B is a schematic cross-sectional view of a printhead die in anexample fluid ejection module having a convex piezoelectric actuator.

Many of the layers and features are exaggerated to better show theprocess steps and results. Like reference numbers and designations inthe various drawings indicate like elements.

DETAILED DESCRIPTION

Fluid droplet ejection can be implemented with a printhead module whichincludes a die fabricated using MEMS processing techniques. Theprinthead die includes a substrate in which a plurality ofmicrofabricated fluid flow paths are formed, and a plurality ofactuators on the substrate to cause fluid to be selectively ejected fromnozzles connected to the flow paths. Each flow path with its associatedactuator provides an individually controllable MEMS fluid ejector unitand the plurality of actuators form an actuator assembly.

A MEMS actuator having a curved piezoelectric membrane can be formedusing a profile-transferring substrate having a curved surface feature(e.g., a dent or a dome in a planar substrate surface). Accordingly, anactuator assembly having an actuator array can be formed using aprofile-transferring substrate having an array of curved surfacefeatures (e.g., an array of dents or domes in a planar substratesurface). The piezoelectric material used for the piezoelectric actuatoris deposited (e.g., sputtered) over at least the curved portions of theprofile-transferring substrate surface before the profile-transferringsubstrate is removed from below the curved portions of the piezoelectricmembrane.

Without being limited to any particular theories, the resulting curvedpiezoelectric membrane can include grain structures that are columnarand aligned in both the curved portions and any planar portionssurrounding the curved portions, and all or substantially all of thecolumnar grains are locally perpendicular to the surface of thepiezoelectric membrane. Such aligned grain structures have less internalstress during actuation and can be more durable than curvedpiezoelectric membranes formed by grinding a bulk piezoelectric materialor by injection molding.

Different processes may be used to form curved features on a profiletransferring substrate surface. As described in this specification, anoxide layer is formed within and around an inverted, truncated pyramidalrecess etched in a semiconductor substrate surface. The top surface ofthe oxide layer (including the curved portions protruding into thesubstrate) can be used to provide a profile-transferring substratesurface having concave surface features (e.g., dents). Alternatively,the oxide layer can be stripped, and the semiconductor substrate surfacebelow the stripped oxide layer can be used to provide aprofile-transferring substrate surface having concave surface features(e.g., dents). Alternatively, the oxide layer is not stripped, instead,a handle layer can be attached to the top surface of the oxide layer,and the substrate can be removed to expose the back side of the oxidelayer. The exposed backside of the oxide layer (including the curvedportions protruding away from the handle layer) can be used to providethe profile-transferring substrate surface having convex surfacefeatures (e.g., domes).

When a thin layer of material is uniformly deposited on theprofile-transferring substrate surface, the thin layer of materialadopts shapes that conform to the profile of the profile-transferringsubstrate surface. Thus, the thin layer of material forms a membranethat has curved features conforming to the curved surface features ofthe profile-transferring substrate surface.

The substrate or handle layer can be subsequently etched from under atleast the curved portions of the membrane, leaving the curved featuresof the membrane suspended and free to deflect. The first thin layer ofmaterial that is deposited over the profile-transferring substratesurface can be a bottom electrode layer of a piezoelectric actuatorassembly. Additional layers of materials (e.g., the piezoelectric layerand the top electrode layer) can be sequentially deposited over thefirst thin layer of material, each adopting curved surface featuresconforming to the curved features of the layer immediately below.

FIGS. 1A-1M illustrate two example processes for forming a membranehaving curved features. FIGS. 1A-1D followed by FIGS. 1E-1G illustratethe example process for making a membrane having concave features (e.g.,dents). FIGS. 1A-1D followed by FIGS. 1H-1M illustrate the exampleprocess for making a membrane having convex features (e.g., domes).

In FIG. 1A, a mask 102 (e.g., a patterned silicon oxide mask, or apatterned silicon nitride mask) is placed or formed on the top surface104 of a planar substrate 100 (e.g., a semiconductor substrate such as asilicon wafer). The planar substrate 100 also includes a planar bottomsurface 108 opposite the top surface 104. The mask 102 includes one ormore openings 106 or an array of openings 106. Each opening 106 can beof a polygonal shape (e.g., a square or rectangle) with straight edges.The polygonal shape of the opening 106 allows flat-walled (e.g.,vertical or slanted) recesses to be formed in the substrate surface 104using anisotropic etching (e.g., KOH etching).

In some implementations, the shape of the opening 106 is chosen based onthe crystal lattice structures of the substrate 100 to form recesseswith flat, slanted sidewalls converging onto a flat, horizontal endwall. In addition, the etchant used in the anisotropic etching hasdifferent selectivity for different crystal planes of the substrate 100,and the geometries of the recesses to be formed in the substrate surface104 can be controlled by the orientation-dependent selectivity of theetchant. Factors that affect the orientation-dependent selectivity ofthe etchant include the composition and concentration of the etchant andthe temperature of the etching environment, for example.

In one example, for a silicon substrate with a <100> surfaceorientation, to form a recess in the shape of an inverted, truncatedpyramid, the opening 106 is square or rectangular. The etchant (e.g., aKOH solution) used in the anisotropic etching has different selectivityfor the different crystal planes of the substrate (e.g., etch rate for{110}> etch rate for {100}>> etch rate for {111}). The resulting recessformed in the substrate surface can have flat sidewalls and a flat endwall, and the geometry of the recess is an inverted, truncated pyramidwith a square or rectangular base. For example, the sidewalls of therecess can be formed in the <111> crystal planes of the siliconsubstrate, while the end wall of the recess can be formed in a <100>crystal plane.

The sizes, locations, and overall distribution of the openings 106 inthe mask 102 can be chosen based on the desired sizes, locations, andoverall distribution of the curved surface features on theprofile-transferring substrate surface to be made, and ultimately, ofthe curved features on the layers of membranes to be formed using theprofile-transferring substrate surface. Adjustments to the lateraldimensions of the openings 106 can be made to accommodate the variationscreated due to the thickness of the different layers (e.g., the oxidelayer) that is to be deposited over the recesses.

For example, for making a profile-transferring substrate with dents inits surface, the size of the opening 106 can be made slightly smallerthan the desired size of the dents to compensate for the subsequentgrowth and removal of the oxide layer on the inside of the recesses.Alternatively, if the oxide layer is not stripped and is to be used toprovide the profile-transferring substrate surface with dents formedtherein, the size of the opening 106 can be made slightly larger thanthe desired size of the dents to compensate for the growth of the oxidelayer on the inside the recesses. Alternatively, if the oxide layer isto be used to provide a profile-transferring substrate with domes formedtherein, the size of the opening 106 can be made slightly smaller thanthe desired size of the domes to compensate for the growth of the oxidelayer on the inside of the recesses.

In FIG. 1B, the mask-covered substrate 100 is exposed to the anisotropicetching (e.g., immersed in a KOH solution with selected concentration ata selected temperature) through the openings 106 of the mask 102, andone or more recesses 110 have been formed in the top surface 104 of thesubstrate 100 at the locations of the openings 106.

Each recess 110 includes a flat end wall 112 and four flat sidewalls114. The sidewalls 114 are each slanted at an angle relative to a planeparallel to the end wall 112. The four sidewalls 114 of each recess 110converge toward the end wall 112 of the recess 110 and are each joinedto the end wall 112 at a respective edge of the end wall 112. Each pairof adjacent sidewalls 114 of each recess 110 are also joined at arespective edge of the recess 110. The top surface 104 of the substrate100 now includes curved portions recessed into the body of the substrate100 (i.e., the portions formed by the sidewalls 114 and end walls 112 ofthe recesses 110) and planar portions 116 surrounding the curvedportions.

In some implementations, the anisotropic etching can be stopped when apredetermined depth of the recess 110 (i.e., as measured by the distancebetween the plane containing the opening of the recess 110 and the planecontaining the end wall 112 of the recess 110) has been reached. In someimplementations, the etch rate can be calibrated in terms of time andetch depth, and the anisotropic etching can be stopped after a timeperiod corresponding to a desired depth of the recess 110 has expired.In some implementations, an silicon-on-insulator (SOI) wafer can be usedas the substrate 100, and the oxide layer in the SOI wafer can serve asan etch stop when etching the recess 110. When an SOI wafer is used, thesilicon layer of the SOI wafer can be thinned (e.g., by polishing) to athickness equal to the desired depth of the recess 110 before theetching is started.

After the desired depth of the recess 110 has been reached, the etchingcan be stopped, and the mask 102 can be removed (e.g., by etching orpolishing) from the top surface 104 of the substrate 100. FIG. 1C showsthe substrate 100 after the recesses 110 have been formed in the topsurface 104 of the substrate 100. At this point, the respectiveintersections between the end wall 112 and each sidewall 114 of eachrecess 110, and the respective intersections between each pair ofadjacent sidewalls 114 of each recess 110, and the respectiveintersections between each sidewall 114 and the planar portion 116 ofthe top surface 104, are all abrupt transitions (as opposed to round orsmooth transitions).

In some implementations, if abrupt transitions between differentportions of a curved feature are acceptable in a membrane to be formed,the top surface 104 as shown in FIG. 1C can be used as aprofile-transferring substrate surface as is. However, in manyapplications, it is desirable to have curved features that do notcontain abrupt transitions between its different facets. Smoother androunded transitions between different facets of a curved feature in amembrane can potentially reduce the likelihood of breakage during use,and improve the lifetime of the MEMS devices containing the membrane.

To form a profile-transferring substrate surface with curved featureshaving rounded corners (i.e., smooth and rounded transitions betweenintersections of adjacent facets of each curved feature), an oxide layer118 can be formed over the top surface 104 of the substrate 100, bothinside the recesses 110 on the sidewalls 114 and end walls 112 of therecesses 110, and on the planar portion 116 of the top surface 104, asshown in FIG. 1D. In some implementations, the oxide layer 118 can begrown on the top surface 104 of the substrate 100 by exposing the topsurface 104 to an oxidizing environment (e.g., an oxygen rich, hightemperature environment). The growth of the oxide layer 118 (e.g.,infusion of oxygen into the surface of the substrate 100) changes thesurface profile of the top surface 104 of the substrate 100, and causesthe abrupt corners (e.g., the corners at the intersections betweendifferent facets of each recess 110) existing in the top surface 104 tobecome smooth and rounded.

As shown in FIG. 1D, an oxide layer 118 has been formed over the topsurface 104 of the substrate 100. The oxide layer 118 include protrudingportions 110′ inside the recesses 110 and a planar portion 116′ on theplanar portion 116 of the top surface 104 of the substrate 100. Eachprotruding or curved portion 110′ of the oxide layer 118 includessidewall portions 114′ on the sidewalls 114 of a corresponding recess110, and an end wall portion 112′ on the end wall 112 of thecorresponding recess 110.

Within each curved portion 110′ of the oxide layer 118, the respectiveintersection 122′ between each sidewall portion 114′ and the adjacentend wall portion 112′ is rounded. In addition, the respectiveintersection between each pair of adjacent sidewall portions 114′ in thecurved portion 110′ is also rounded. The respective intersectionsbetween the planar portion 116′ and the curved portion 110′ of the oxidelayer 118 are also rounded.

In some implementations, the top surface 120 of the oxide layer 118(including both the planar portions 116′ and the curved portions 110′protruding into the recesses 110) can be used as a profile-transferringsubstrate surface for making a membrane with dents formed therein. Thethickness of the oxide layer 118 can be between a few microns (e.g., 1-3microns). The depth of the curved portion 110′ of the oxide layer 118can be between 5-10 microns, while the lateral dimensions of the openingof the curved portion 110′ can be a few hundred microns (e.g., 100-200microns).

In some implementations, the oxide layer 118 can be stripped from thetop surface 104 of the substrate 100. For example, the oxide layer 118can be stripped by immersing the structure shown in FIG. 1D in an oxidestripping solution (e.g., a Dilute Hydrofluoric Acid (DHF) solution)briefly. As shown in FIG. 1E, after the oxide stripping, the top surface104 of the substrate 100 is exposed, and the top surface profile of thesubstrate 100 exhibits rounded corners at the intersections 122 betweenthe sidewalls 114 and the end wall 112 of each recess 110, at theintersections between the adjacent sidewalls 114 of the recess 110, andat the intersections between the planar portion 116 and the sidewalls114 of the recess 110. After the oxide stripping, the outer surface 104of the substrate 100 can be used as a profile-transferring substratesurface for making a membrane having dents formed therein, where theintersections between different facets of each recess 110 are roundedand smooth.

As shown in FIG. 1F, a thin layer of material 124 is deposited on theexposed top surface 104 of the substrate 100 after the oxide stripping.In some implementations, the thin layer of material 124 can be a layerof conductive material (e.g., a few thousands of Angstroms of Au, Au/W,Ir, Pt, and so on). The layer of material 124 is of uniform thicknessand can serve as a bottom electrode layer of a piezoelectric actuatorassembly. The layer of conductive material 124 includes a planar portionon the planar portion 116 of the top surface 104, and protruding orcurved portions 126 over the recesses 110 in the top surface 104. Thus,the thin layer of material 124 is a membrane including planar portionsand curved portions 126 protruding into the body of the substrate 100.In some implementations, after the oxide layer 118 is stripped, anotherlayer of oxide can be formed (e.g., grown or deposited) on the exposedtop surface 104 of the substrate 100, and the newly formed layer ofoxide is a membrane including planar portions and curved portions 126protruding into the body of the substrate 100. The newly formed oxidelayer can be thinner than the oxide layer 118, such that theintersections between the different portions of the newly formed oxidelayer remain smooth and rounded at the completion of the growth ordeposition of the new oxide layer.

The respective intersections between the planar portion and the curvedportions 126 of the membrane 124 conform to the rounded intersectionsbetween the planar portion 116 and the recesses 110 in the top surface104, and are also smooth and rounded. In addition, the respectiveintersections between the sidewalls and the end wall of each curvedportion 126 and the respective intersections between adjacent sidewallsof each curved portion 126 are also smooth and rounded, as are thecorresponding intersections in the recesses 110 of the substrate 100.

Additional layers of materials (not shown in FIG. 1F) can be depositedover the top surface 128 of the first layer of material 124, such as apiezoelectric layer, and a top electrode layer. Various suitable methodsfor depositing the one or more layers of materials can be used. Forexample, the bottom electrode layer, the piezoelectric layer, and thetop electrode layer can each be deposited by sputtering, plasma-enhancedvapor deposition, chemical vapor deposition, or physical vapordeposition, and so on. The different layers of materials can each bepatterned before the next layer of material is deposited over it. Thedifferent layers can form the piezoelectric actuator assembly of a fluidejection module or other MEMS devices.

In some implementations, the substrate 100 or at least a portion of itcan be removed from the backside (i.e., from the back surface 108 of thesubstrate 100) to expose the curved portions 126 of the membrane 124.For example, the substrate material is removed (e.g., by etching) fromareas of the bottom surface 108 of the substrate 100 that are directlybelow the curved portions 126 of the membrane 124. A cavity can beformed in the substrate 100 on the bottom side of the substrate 100. Theetching can continue within the cavity until the bottom surface of themembrane 124 becomes exposed.

As shown in FIG. 1G, in some implementations, an area of the substrate100 is etched away to form a cavity 130. The lateral dimensions of thecavity 130 can be smaller than the lateral dimensions of the entirecurved portions 126 of the membrane 124, thus, only the lower portionsof the curved portions 126 (e.g., including the end wall and the lower ¾of the sidewalls of the curved portions 126) are exposed within thecavity 130, leaving the upper portions of the curved portion 126embedded within the substrate 100, as shown in FIG. 1G.

In some implementations, the cavity 130 can have larger lateraldimensions, and the entire curved portions 126 (e.g., including the endwall and all of the sidewalls) of the membrane 124 can be exposed byetching within the cavity 130. In some implementations, the lateraldimensions of the cavity 130 can be made even larger, exposing theentire curved portions 126 of the membrane 124 and part of the planarportions of the membrane 124 surrounding the curved portions 126.

Although not shown in FIGS. 1E-1G, in some implementations, after theoxide layer 118 is stripped following the step shown in FIG. 1D, a newlayer of oxide can be formed (e.g., deposited or grown) on the exposedsubstrate surface 104. The newly formed oxide layer (not shown) can beused as the profile-transferring substrate surface for forming themembrane 124, when the cavities 130 are created (e.g., by etching) inthe back side 108 of the substrate 100, the backside of the newly formedoxide layer is exposed first. In some implementations, the etching canbe stopped after the backside of the newly formed oxide layer isexposed, and the exposed back surface of the oxide layer 118 can serveas an etch stop for the etching process. Alternatively, the etching cancontinue to thin the oxide layer 118 to a desired thickness within thewalls of the cavity 130. By reducing the thickness of the newly formedoxide layer, the membrane 124 (e.g., a bottom electrode layer) and anyadditional layers (e.g., the piezoelectric layer and the top electrodelayer) formed above the membrane 124 can have better compliance anddeform more freely under pressure or other actuating forces (e.g.,driving voltages). The thinned, newly formed oxide layer can protect themembrane 124 from being eroded or contaminated by other materials (e.g.,ink) that would fill the cavity 130 in an actual MEMS device (e.g., aprinthead). In some implementations, the thickness of the newly formedoxide layer is chosen to be thinner than the oxide layer 118, such thatthe newly formed oxide layer is sufficiently flexible and hassufficiently smooth and rounded transitions between its differentportions. In some implementations, the etching of the oxide layer 118can continue until the back surface of the membrane layer 124 becomesexposed within the cavities 130. In such implementations, the newlyformed oxide layer is completely removed from the areas within the wallsof the cavities 130 in the bottom side 108 of the substrate 100, butportions of the oxide layer 118 remain at least between the planarportions of the substrate 100 and the planar portions of the membrane124.

The process described with respect to FIGS. 1A-1G and the resultingstructure shown in FIG. 1G can be used to form an example fluid ejectionsystem 200 a shown in FIG. 2A. More detail on the example fluid ejection200 a shown in FIG. 2A will be provided later in the specification.

FIGS. 1A-1D followed by FIGS. 1H-1M illustrate an example process formaking a profile-transferring substrate surface having convex surfacefeatures (e.g., domes) formed therein, and making a membrane havingconvex features using the profile-transferring substrate surface.

To make the profile-transferring substrate surface having domes formedtherein, a recess is first formed in the planar top surface 104 of thesubstrate 100, and an oxide layer 118 is formed on the top surface 104of the substrate 100, according to the process described above withrespect to FIGS. 1A-1D. The oxide layer 118 has a planar portion 116′above the planar portion 116 of the substrate surface 104, and curvedportions 110′ (i.e., portions formed by the sidewall portions 114′ andthe adjacent end wall portion 112′ of the oxide layer 118) over thesidewalls 114 and the end wall 112 of the recesses 110. The curvedportions 110′ of the oxide layer 118 protrude into the body of thesubstrate 100. Since the oxide layer 118 is a layer of uniformthickness, when another substrate (e.g., a handle layer) is attached tothe top surface 120 the oxide layer 118, and the entire structure isflipped over, the concave curved portions 110′ of the oxide layer 118will become convex curved portions relative to the body of the newsubstrate.

As shown in FIG. 1H, a planar handle layer 132 (e.g., another siliconsubstrate) is placed over the top surface 120 of the oxide layer 118shown in FIG. 1D. The handle layer 132 can be a few hundred micronsthick (e.g., 150-600 microns). In some implementations, before theplanar bottom surface 134 of the handle layer 132 makes contact with thetop surface 120 of the oxide layer 118, the bottom surface 134 of thehandle layer 132 and the top surface 120 of the oxide layer 118 can beprepared (e.g., cleaned and polished) for bonding.

As shown in FIG. 1I, when the bottom surface 134 of the handle layer 132and the top surface 120 of the oxide layer 118 are pressed together,only the planar portion 116′ of the oxide layer 118 make contact withthe planar bottom surface 134 of the handle layer 132. In someimplementations, the handle layer 132 and the oxide layer 118 areannealed, such that the temporary bond between the bottom surface 134 ofthe handle layer 132 and the top surface 120 of the oxide layer 118becomes a permanent bond, and the openings of the curved portions 110′become sealed by the bottom surface 134 of the handle layer 132.

When the structure shown in FIG. 1I is flipped over, the curved portions110′ of the oxide layer 118 become convex features relative to thehandle layer 132. The substrate 100 can be removed (e.g., by grindingand etching) from the bottom side 108 to expose the embedded backsurface of the oxide layer 118. The exposed back surface of the oxidelayer 118 will include curved portions protruding away from the handlelayer surrounded by the planar portion of the oxide layer 118.

FIG. 1J shows that the substrate 100 has been removed (e.g., by grindingand etching) such that the oxide layer 118 that previously covers therecesses 110 in the substrate 100 now become exposed from the backside.The previously embedded lower surface of the oxide layer 118 is now theexposed outer surface 138 of the oxide layer 118. The exposed oxidelayer 118 includes the planar portion 116′ and curved portions 110″protruding away from the handle layer 132. The protruding or curvedportions 110″ include a flat end wall 112′, and slanted sidewalls 114′,and are in the shape of a truncated pyramid with a rectangular base. Theconcave curved portions 110′ of the oxide layer 118 now becomes convexcurved portions 110″ (e.g., oxide dome shells).

In some implementations, when the substrate 100 is removed by etching,the oxide layer 118 can serve as the etch stop during the removal of thesubstrate 100. The intersections 122″ between the end wall 112′ and thesidewalls 114′ of each oxide dome shell 110″, and the intersectionsbetween adjacent sidewalls 114′ of the oxide dome shell 110″ arerounded. The exposed outer surface 138 of the oxide layer 118 canprovide the profile-transferring substrate surface having convex surfacefeatures (e.g., domes) formed therein.

As shown in FIG. 1K, a thin layer of material 140 is deposited on theexposed outer surface 138 of the oxide layer 118 (e.g., over at leastthe areas of the oxide dome shells 110″). In some implementations, thethin layer of material 140 can be a layer of conductive material (e.g.,a few thousands of Angstroms of Au, Au/W, Ir, Pt, and so on). The layerof material 140 is of uniform thickness and can serve as a bottomelectrode layer of a piezoelectric actuator assembly. The layer ofconductive material 140 includes a planar portion on the planar portion116′ of the oxide layer 118, and curved portions 142 on the curvedportions 110″ of the oxide layer 118. Thus, the thin layer of material140 is a membrane including a planar portion and curved portions 142protruding away from the body of the handle layer 132.

The intersections between the planar portion and the curved portions 142of the membrane 140 conforms to the rounded intersections between theplanar portion 116′ and the curved portions 110″ of the oxide layer 118,and are also smooth and rounded. In addition, the respectiveintersections between the sidewalls and the end wall of each curvedportion 142 and the respective intersections between adjacent sidewallsof each curved portion 142 of the membrane 140 are also smooth androunded, as are the corresponding intersections in the curved portion110″ of the oxide layer 118.

Additional layers of materials (not shown in FIG. 1K) can be depositedover the top surface of the first layer of material 140, such as apiezoelectric layer, and a top electrode layer. Various suitable methodsfor depositing the one or more layers of materials can be used. Forexample, the bottom electrode layer, the piezoelectric layer, and thetop electrode layer can each be deposited by sputtering, plasma-enhancedvapor deposition, chemical vapor deposition, or physical vapordeposition, and so on. The different layers of materials can each bepatterned before the next layer of material is deposited over it. Thedifferent layers can form the piezoelectric actuator assembly of a fluidejection module or other MEMS devices.

In some implementations, the handle layer 132 or at least a portion ofit can be removed from the backside (i.e., from the back surface of thehandle layer 132 opposite to the surface 134 shown in FIG. 1H) to exposeat least the curved portions 110″ of the oxide layer 118 from the backsurface (i.e., previously the top surface 120 of the oxide layer 118shown in FIG. 1I). For example, as shown in FIG. 1L, the handle layermaterial is removed (e.g., by etching) from areas of the bottom surfaceof the handle layer 132 that are directly below the curved portions 110″(i.e., the portion 110″ formed by the sidewalls 114′ and the adjacentend wall 112′) of the oxide layer 118. As a result, a cavity 144 can beformed in the handle layer 132 on the bottom side of the handle layer132. The etching can continue within the cavity 144 until the backsurface (i.e., previously the top surface 120) of the oxide layer 118become re-exposed or opened to atmosphere.

In some implementations, the etching can be continued until the oxidelayer 118 is thinned to a desired thickness within the walls of thecavity 144. By reducing the thickness of the oxide layer 118, themembrane 140 (e.g., a bottom electrode layer) and any additional layers(e.g., the piezoelectric layer, and the top electrode layer) formedabove the membrane 140 can have better compliance and deform more freelyunder pressure or other actuating forces (e.g., driving voltages). Thethinned oxide layer 118 can protect the membrane 140 from being erodedor contaminated by other materials (e.g., ink) that would fill thecavity 144 in an actual MEMS device (e.g., a printhead).

In some implementations, the etching of the oxide layer 118 can becontinued until the back surface of the membrane layer 140 becomesexposed within the cavities 144, in other words, until the inner surfaceof the curved portions 142 in the membrane layer 140 become exposed oropened to atmosphere. As shown in FIG. 1M, the oxide layer 118 has beencompletely removed from the areas within the sidewalls of the cavities144 in the bottom side of the handle layer 132. The oxide layer 118remains between the planar portions of the handle layer 132 and theplanar portions of the membrane 140. In some implementations, thecavities 144 can be made larger, and the entire curved portions 142 ofthe membrane 140 and some planar portion of the membrane 140 surroundingthe curved portions 142 can be exposed and suspended within the cavities144.

The process described with respect to FIGS. 1A-1D followed by FIGS.1H-1M and the resulting structure shown in FIG. 1M can be used to forman example fluid ejection system 200 b shown in FIG. 2B. More detail onthe example fluid ejection 200 b shown in FIG. 2B will be provided laterin the specification.

FIG. 2A is a schematic of an example fluid ejection system 200 a thatcan be formed at least in part using the process shown in FIG. 1A-1G. Asshown in FIG. 2A, the substrate 100 having the cavities 130 formedtherein can serve as a pumping chamber layer 202 a of the fluid ejectionsystem 200 a, and the cavities 130 can serve as the pumping chambercavities 204 a for the fluid ejection system 200 a. The pumping chambercavities 204 a are connected to fluid paths that have been formed in thepumping chamber layer 202 a in a different process. Nozzles 206 a areformed in a nozzle layer 208 a, and are connected to the pumping chambercavities 204 a. A number of layers that are deposited over the substrate100 after the oxide layer 118 was stripped can form the piezoelectricactuator assembly 210 a above the pumping chamber layer 202 a. As shownin FIG. 2A, the layers include a bottom electrode layer 212 a, apiezoelectric layer 214 a, and a top electrode layer 216 a. Each ofthese three layers can be patterned to define individual actuator unitsthat include a top electrode, a bottom electrode, and piezoelectricelement directly above each pumping chamber cavity 204 a. Each of thesethree layers include respective curved portions protruding into theapertures of the pumping chamber cavities 204 a, surrounded by arespective planar portion.

Although not shown in FIG. 2A, in some implementations, if thepiezoelectric assembly 210 a had been formed over an oxide layerdeposited or grown on the substrate surface after the oxide layer 118was stripped, and if the newly formed oxide layer is thinned but notcompletely removed within the walls of the cavities 130, the newlyformed oxide layer can exist in areas within the sidewalls of thepumping chamber cavities 204 a with a smaller thickness, but stillexists with its original thickness in the planar portions of the pumpingchamber layer 202 a outside of the pumping chamber cavities 204 a. Thecurved portions of the newly formed oxide layer protrude into theapertures of the pumping chamber cavities 204 a, support the curvedportions of the layers deposited above the newly formed oxide layer. Insome implementations, the newly formed oxide layer can be completelyremoved from within the sidewalls of the cavities 204 a, but exist withits original thickness in the planar portions of the pumping chamberlayer 202 a. In some implementations, the newly formed oxide layer canmaintain its original uniform thickness in all areas, existing below andsupporting the actuator assembly 210 a.

FIG. 2B is a schematic of an example fluid ejection system 200 b thatcan be formed at least in part using the process shown in FIG. 1A-1D,followed by FIGS. 1H-1M. As shown in FIG. 2B, the oxide layer 118 inconjunction with the handle layer 132 having the cavities 142 formedtherein, can serve as a pumping chamber layer 202 b of the fluidejection system 200 b, and the cavities 142 can serve as the pumpingchamber cavities 204 b for the fluid ejection system 200 b. The pumpingchamber cavities 204 b are connected to fluid paths that have beenformed in the pumping chamber layer 202 b in a different process.Nozzles 206 b are formed in a nozzle layer 208 b, and are connected tothe pumping chamber cavities 204 b.

A number of layers that are deposited over the oxide layer 118 can formthe piezoelectric actuator assembly 210 b above the pumping chamberlayer 202 b. As shown in FIG. 2B, the layers include a bottom electrodelayer 212 b, a piezoelectric layer 214 b, and a top electrode layer 216b. Each of these three layers can be patterned to define individualactuator units that include a top electrode, a bottom electrode, andpiezoelectric element directly above each pumping chamber cavity 204 b.In some implementations, the oxide layer 118 has uniform thickness inall areas. In some implementations, the oxide layer 118 has been thinnedin areas within the sidewalls of the pumping chamber cavities 204 b, butstill has its original thickness in portions outside of the pumpingchamber cavities 204 b. In some implementations, the oxide layer 118 canbe completely removed within the sidewalls of the cavities 204 b, butstill exists with its original thickness in portions outside of thepumping chamber layer 202 b.

Although examples are described in terms of a process for making apiezoelectric actuator assembly for a fluid ejection system, the processcan be used in making other MEMS devices that include membranes havingcurved features or arrays of curved features.

The use of terminology such as “front,” “back,” “top,” “bottom,” “over,”“above,” and “below” throughout the specification and claims is toillustrate the relative position or orientation of various components ofthe system. The use of such terminology does not imply a particularorientation of the structure. Similarly, the use of any horizontal orvertical terms to describe elements is in relation to the implementationdescribed. In other implementations, the same or similar elements can beoriented other than horizontally or vertically as the case may be.

What is claimed is:
 1. A process for forming a membrane having a curvedfeature, comprising: oxidizing a top surface of a semiconductorsubstrate to form an oxide layer, the top surface of the semiconductorsubstrate having a recess formed therein, the recess having a flat endwall and multiple flat, slanted sidewalls adjacent to the end wall, theoxide layer including a first portion on the end wall of the recess, asecond portion on the slanted sidewalls of the recess, and a planarthird portion on the top surface of the semiconductor substrate aroundan opening of the recess in the top surface of the semiconductorsubstrate; and stripping the first, second, and third portions of theoxide layer to leave respective rounded intersections between the endwall and each slanted sidewall of the recess and between adjacentsidewalls of the recess.
 2. The process of claim 1, further comprising:after the stripping, depositing a layer of a first material on the topsurface of the semiconductor substrate, the layer of the first materialcovering at least the end wall and the multiple slanted sidewalls of therecess and the respective rounded intersections; and removing at least aportion of the semiconductor substrate from a bottom surface of thesemiconductor substrate to expose a bottom surface of the layer of thefirst material in at least areas previously covering the end wall of therecess and a lower portion of each sidewall adjacent to the end wall. 3.The process of claim 1, further comprising: etching the recess in thetop surface of the semiconductor substrate through a mask using ananisotropic etchant.
 4. The process of claim 3, further comprising:prior to the etching, forming the mask on the top surface ofsemiconductor substrate, the mask having an opening through which thetop surface of the semiconductor substrate is exposed, and the openingin the mask having multiple straight edges.
 5. The process of claim 4,wherein the opening in the mask is of a rectangular shape.
 6. Theprocess of claim 3, wherein the anisotropic etchant is KOH.
 7. A processfor forming a membrane having a curved feature, comprising: oxidizing atop surface of a semiconductor substrate to form an oxide layer, the topsurface of the semiconductor substrate having a recess formed therein,the recess having a flat end wall and multiple flat, slanted sidewallsadjacent to the end wall, the oxide layer including a first portion onthe end wall of the recess, a second portion on the slanted sidewalls ofthe recess, and a third portion on the top surface of the semiconductorsubstrate around an opening of the recess in the top surface of thesemiconductor substrate; bonding a handle layer to a first side of theoxide layer, the handle layer in contact with the third portion of theoxide layer and not in contact with the first portion of the oxidelayer; and removing at least part of the semiconductor substrate toexpose a second side of the oxide layer opposite to the first side, inareas of the first portion and at least part of the second portionadjacent to the first portion of the oxide layer.
 8. The process ofclaim 7, further comprising: depositing a layer of a first material onthe second side the oxide layer over at least the first portion and theat least part of the second portion of the oxide layer.
 9. The processof claim 8, further comprising: after depositing the layer of the firstmaterial, removing at least part of the handle layer to re-expose thefirst side of the oxide layer in at least the first portion and thesecond portion of the oxide layer.
 10. The process of claim 9, furthercomprising: after removing at least the part of the handle layer,removing at least the first portion and the at least part of the secondportion of the oxide layer to expose a portion of the layer of the firstmaterial previously deposited on the removed portions of the oxidelayer.
 11. The process of claim 7, further comprising: etching therecess in the top surface of the semiconductor substrate through a maskusing an anisotropic etchant.
 12. The process of claim 11, furthercomprising: prior to the etching, forming the mask on the top surface ofsemiconductor substrate, the mask having an opening through which thetop surface of the semiconductor substrate is exposed, and the openingin the mask having multiple straight edges.
 13. The process of claim 12,wherein the opening in the mask is of a rectangular shape.
 14. Theprocess of claim 11, wherein the anisotropic etchant is KOH.
 15. Aprocess for forming a membrane having a curved feature, comprising:oxidizing a top surface of a semiconductor substrate to form an oxidelayer, the top surface of the semiconductor substrate having a recessformed therein, the recess having a flat end wall and multiple flat,slanted sidewalls adjacent to the end wall, the oxide layer including afirst portion on the end wall of the recess, a second portion on theslanted sidewalls of the recess, and a third portion on the top surfaceof the semiconductor substrate around an opening of the recess in thetop surface of the semiconductor substrate; stripping the oxide layer toleave respective rounded intersections between the end wall and eachslanted sidewall of the recess and between adjacent sidewalls of therecess; after the stripping, depositing a layer of a first material onthe top surface of the semiconductor substrate, the layer of the firstmaterial covering at least the end wall and the multiple slantedsidewalls of the recess and the respective rounded intersections; andremoving some of the semiconductor substrate from a bottom surface ofthe semiconductor substrate to expose a bottom surface of the layer ofthe first material in at least areas previously covering the end wall ofthe recess and a lower portion of each sidewall adjacent to the endwall, wherein, when the some of the semiconductor substrate is removed,at least a portion of the semiconductor substrate remains betweenadjacent exposed bottom surfaces of the layer of the first material inareas previously covering the end wall of the recess.
 16. A process forforming a membrane having a curved feature, comprising: oxidizing a topsurface of a semiconductor substrate to form an oxide layer, the topsurface of the semiconductor substrate having a recess formed therein,the recess having a flat end wall and multiple flat, slanted sidewallsadjacent to the end wall, the oxide layer including a first portion onthe end wall of the recess, a second portion on the slanted sidewalls ofthe recess, and a third portion on the top surface of the semiconductorsubstrate around an opening of the recess in the top surface of thesemiconductor substrate; bonding a handle layer to a first side of theoxide layer, the handle layer in contact with the third portion of theoxide layer; removing at least part of the semiconductor substrate toexpose a second side of the oxide layer opposite to the first side, inareas of the first portion and at least part of the second portionadjacent to the first portion of the oxide layer; and after depositing alayer of a first material, removing at least part of the handle layer tore-expose the first side of the oxide layer in at least the firstportion and the second portion of the oxide layer.